I. Field of the Invention
The present invention relates to a metallurgical furnace for liquid phase sintering of preforms of powdered metals, ceramics, and the like.
II. Description of the Prior Art
In the liquid phase sintering of preforms of powdered metals, ceramics, and the like, the preforms are sequentially subjected to a presinter step, a sinter step and at times a hot isostatic pressing (HIP). Each of these steps is discussed separately below.
The presinter step is used primarily to remove the fugitive binder or "wax" which holds the part which is constructed of the powdered material in the desired shape after cold pressing. Usually this fugitive binder consists of a paraffin, polyetheleneglycol or a metal containing a hydrocarbon. The presinter step is also known as the "dewax" step.
During the presintering step, the cold pressed parts are slowly raised through the vaporization temperature of the various molecular weight constituents of the hydrocarbons used in the fugitive binder. As these various constituents are vaporized, they are evacuated from the furnace and thus from the parts by a wash gas, such as hydrogen, or removed by vacuum pumping. When vacuum pumping is used to remove the vapors of the fugitive binder, a condensation means is provided between the furnace and the vacuum pump to prevent the wax vapors from entering and possibly damaging the vacuum pump.
During the presinter operation, the vapors of the fugitive binder, or wax vapors, must be removed from the furnace before the "cracking" temperatures of the various hydrocarbon constituents of the wax vapors are reached. Otherwise, if cracking should occur, carbon deposits on and is absorbed by the parts. Carbon deposits, in the later sintering operation, can cause undesirable carbon changes in the parts.
The final temperature that the parts are subjected to during the presinter operation is approximately 400.degree. C.
In the previously known vacuum furnaces capable of presintering, relatively thin and porous insulation is provided around the furnace chamber, i.e., the chamber in which the parts are processed. The furnace chamber, in turn, is contained within a pressure and/or vacuum vessel (hereinafter collectively referred to as a pressure vessel). This relatively thin insulation is designed so that the outside of the insulation becomes sufficiently heated to prevent condensation of the wax vapors on the outside of the insulation. Otherwise, during the sintering operation, these wax vapors will revaporize and contaminate the parts.
There have been previously known furnaces which perform both the presinter and sintering steps or operations. by raising the temperature of the furnace to about 1,200.degree.-2,000.degree. C. following the dewax step. In one type of previously known stoking furnaces, a flowing cover gas, such as hydrogen, is continually supplied to the furnace chamber, while in another type of previously known furnace, the furnace chamber is evacuated during the sintering operation. The parts are kept within the furnace chamber for a sufficient time for the parts to obtain proper densification and microstructural development. If the porosity levels of the parts are acceptable following the sintering operation, the parts can be then finished and used.
Since these previously known furnaces which perform both the presinter and sinter operations use relatively thin and porous insulation around the furnace chamber in order to prevent wax condensation on the insulation during the presinter step, a great amount of heat loss from the furnace chamber occurs during the sintering operation in view of the higher temperatures required during sintering. Such large heat losses not only result in high power consumption from the furnace heating elements but also necessitates large cooling requirements for the cooling of the pressure vessel surrounding the furnace chamber.
Following the sintering step, the parts are subjected to HIP processing if further densification of the parts is required. In HIP processing, the parts are loaded into graphite containers and placed within the furnace chamber of the HIP equipment. The furnace chamber is then evacuated and, while still cold, pressurized to approximately 5,000 psi with an inert gas, such as argon. The temperature of the furnace compartment is then raised to the liquid phase region of the parts, typically 1,200.degree.-1,500.degree. C., and the thermal expansion of the argon gas increases the pressure to approximately 10,000-15,000 psi. Under these conditions, porosity and voids within the parts are effectively closed.
Due to the high temperatures and pressures used during HIP processing, the previously known HIP equipment is extremely massive in construction and expensive to acquire. Although the HIP equipment could be used to both perform the sintering step and HIP step, HIP equipment is not designed for and, therefore, cannot be used to dewax the preforms, i.e., perform the presintering step. For example, HIP equipment does not include means for condensing or collecting wax from the preforms. In any event, it would not be cost effective to perform the presintering step in the HIP equipment since presintered parts occupy approximately twice the volume of sintered parts so that the capacity of the HIP equipment would not be effectively used.
In a still further type of metallurgical sinter furnace, the outside walls of the furnace are directly cooled by a water jacket. This furnace cannot be used to perform the dewax step since, during sinter, the wax would revaporize and contaminate the parts.
None of the previously known metallurgical furnaces have been capable of performing the presinter, sintering and HIP processing within a single furnace chamber. Consequently, the parts must necessarily be transferred between at least two and usually three separate furnaces or furnace chambers in order for the entire processing of the part from presinter and through the HIP to be completed. Since the parts must usually be cooled to room temperature before such transfers any dissolved contaminants, such as oxygen, calcium, sulpher and the like, can collect in any porosity or voids in the parts. Such contaminants may require a higher pressure in the HIP step in order to reduce porosity and close voids in the parts. Furthermore, the previously known furnaces which use hydrogen are prone to explosions which presents a safety hazard.